نوع مقاله : مقاله پژوهشی
نویسندگان
1 عضو هیات علمی پژوهشکده هواشناسی
2 عضو هیات علمی گروه جغرافیای طبیعی دانشکده علوم زمین، دانشگاه شهید بهشتی، تهران، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Sudden stratospheric warming (SSW) events are frequently observed over the arctic and are exceptionally rare in the southern hemisphere. A major strong sudden stratospheric warming occurred in the southern hemisphere stratosphere in winter of 2002. The strongest SSW in the northern hemisphere was observed in winter of 2009. During these two phenomena, strong westerly winds associated with the polar vortex decrease, and temperatures a rapid increase in the polar caps. A majority of SSW episodes conclude with either a split or displacement of the polar vortex. These changes in the vortex are significant because they assist to determine the severity of warmings, which can affect regional weather, and can aid in the diagnosis of wave forcing’s. These can be studied effectively using the potential vorticity maps. We used the Ertel’s potential vorticity on the five isentropic levels between, 420 and 850K.
We used the daily zonal mean data from the MERRA2 assimilated data and NCEP-NCAR reanalysis data. From MERRA2, daily zonal-mean zonal wind at 60° and average of zonal-mean temperature at polar cap were obtained at 6 level pressure from 150hPa to 10hPa from 15 November 2008 to 15 February 2009 and 15 June to 15 November 2002. From the NCEP–NCAR data, daily zonal and meridional wind component, temperature and geopotential height at 17 level pressure from 1000hPa to 010hPa were used and computed Ertel’s potential vorticity on the isentropic surfaces. Then scaled the potential vorticity fields to vorticity units as in Dunkerton and Delisi (1986) and was estimated modified potential vorticity. This scaling was performed to provide fields with a similar order of magnitude throughout the stratosphere as opposed to potential vorticity.
We first examined the time evolution cross section of zonally averaged temperature, zonal wind, static stability, absolute vorticity, Ertel’s potential vorticity over pressure surfaces and modify potential vorticity over isentropic surfaces to identify the warmings onset day, zero day and decay day for the trace of SSW and its relation with polar vortex. Then determined three periods: before and after the zero day and the time of recovery. In the next, we examined Ertel potential vorticity changes on different isentropic surfaces in the three periods introduced.
The results present that the Ertel’s potential vorticity in two cases of SSW due to the conservation factor or vertical profile of the potential temperature, was able to show the effect of SSW on the polar vortex, and the changes in the modified potential vorticity at different isentropic levels also revealed the SSW in the southern and northern hemispheres. In addition, the maximum (minimum) value of the modified potential vorticity in the northern (southern) hemisphere was estimated on the isentropic level of 600 K before the zero day of the selected SSW, and it was found that the location of the maximum and minimum cores in both hemispheres were in the range of 70 to 90 degrees north and south latitudes. Also, in the days before the warming, the minimum values of the Ertel’s potential vorticity at different isentropic levels in the 2002 SSW, were greater than the maximum values related to the 2009 SSW, regardless of their sign. Analysis of the minimum and maximum value of Ertel’s potential vorticity at the 600 K isentropic level showed that after the central day or zero day in the SSW of 2002, the position of minimum value gradually moved to the south pole, while the position of the maximum value in the SSW of 2009 shifted to areas further away from the north pole.
کلیدواژهها [English]
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